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Linking Rac1 complexes back to mechanisms of cell-cell
adhesion
The identity of proteins in these Rac1 complexes, and hence the mechanism
of their action on cell migration and cell-cell adhesion, remains unknown.
Since activation of Rac1 increases upon cadherin engagement (Noren et al.,
2001), it could be physically localized to cell-cell contacts by binding specific
proteins (see below), or activity of Rac1 could be controlled by local
recruitment of guanine exchange factors (GEFs; reviewed in Hall, 1998;
Evers et al., 2000; Fukata and Kaibuchi, 2001). Note that many GEFs
localize to membranes through a plekstrin homology domain that binds
phospholipid products of PI 3-kinase (Hurley and Meyer, 2001; Martin,
2001), and that PI 3-kinase is also activated by cadherin engagement
(Nakagawa et al., 2001) and may bind E-cadherin directly (Nakagawa et al.,
2001; Espada et al., 1999; Pece et al., 1999; Woodfield et al., 2001). Indeed,
we found that a fusion protein containing the PH domain of AKT and GFP
[a marker for PI(3,4,5)P 3 and PI(3,4)P 2 ] is rapidly and precisely localized to
the edges of the expanding MDCK cell-cell contacts where Rac1 and
lamellipodia are active (Ehrlich et al., 2002). Inhibitors of PI-3 kinase have
been shown to prevent activation of Rac1 and Cdc42 in response to
cadherin-based adhesion. However, inhibition of PI 3-kinase activity does
not appear to block cell-cell adhesion (Nakagawa et al., 2001; Ehrlich et al.,
2002) suggesting that cell-cell adhesion involves more than one mechanism
working in parallel.
The discrepancy in these results may be explained by the effect of PI3 kinase
inhibition on Rac1 complexes. Contacting MDCK cells treated with PI-3
kinase inhibitors assemble Rac1 complexes A and B, but not complex C. This
indicates that complex C is dispensable for cell-cell contact formation. If
active Rac1 is displaced from complexes B and C, as in cells expressing
dominant negative Rac1, however, formation of cell-cell contacts is disrupted.
We conclude that active Rac1 in complexes B and C perform an identical
function, that of initiating the protrusion of lamellipodia at the plasma
membrane. Coincidence of Rac1GFP and lamellae in live cell imaging studies
supports this conclusion. The observation by Ehrlich et al. (2002) that
dominant negative Rac1 does not accumulate at nascent cell-cell contacts,
while constitutively active Rac1 does, indicates that Rac1 complexes B and C
might have distinct diffusive properties in the plane of the membrane. Thus,
the transition of Rac1 from complex B to complex C might reflect a molecular
mechanism of orientating Rac1 activity to sites of cell-cell adhesion. Given the
results with inhibitors of PI-3 kinase, such a mechanism appears to be
mediated by the generation of specific phospholipids. Future studies that
identify the link between Rac1 localization and phospholipid generation are
ongoing.
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